In a liquid crystal display technology, existing thin film field effect transistors may be of several types, including low temperature poly-silicon (LTPS) transistor, metal-oxide-semiconductor field effect transistor (MOSFET) and amorphous-silicon (a-Si) thin film transistor. During the design of these thin film field effect transistors, a non-uniform driving current is generated by a pixel circuit of a display due to the existence of a threshold voltage and a leakage current, and this poses a challenge to the uniformity of brightness. Hence, how to reduce or compensate for the threshold voltage becomes a key point for designing a pixel driving circuit on a display panel.
Especially for an LTPS technology, as a process for manufacturing a new-generation thin film transistor-liquid crystal display (TFT-LCD) and an important branch of polysilicon technology, it is more urgent to address the above problem due to its characteristics. For the LCD, the LTPS technology can remarkably improve the carrier mobility, so it is able to miniaturize TFT elements, improve an aperture ratio of the panel, increase the brightness and reduce the power consumption. In addition, a glass substrate may be used during the low-temperature manufacturing process, so it is able to remarkably reduce the production cost. As a result, the LTPS technology has been widely used in the manufacturing of the modern LCD.
FIG. 1 shows drain current-gate voltage characteristic curves for a typical N-type MOSFET, a LTPS TFT and an a-Si TFT. The a-Si TFT has a low leakage current, but its on-state current is not high enough due to the low carrier mobility, and obviously an on-state current of the LTPS TFT is much higher than that of the a-Si TFT. When in a cut-off state, the leakage current of the LTPS TFT is increased along with an increase in the negative gate voltage, and there is still a way of electric leakage for the pixel in the cut-off state, resulting in a decrease in the voltage of a storage capacitor in the pixel driving circuit of the display. As a result, in the design of the pixel circuit using the LTPS technology, various methods are used to reduce the leakage current.
However, in the prior art, there is no scheme capable of improving the structure of the thin film field effect transistor so as to affect its current characteristics, thereby to reduce the threshold voltage and the leakage current.